Pruthvi Loy, Chiranth B. P. 1 SJEC, Mangaluru
4. SOLIDIFICATION
Introduction
Mechanism of solidification
- crystallization and development of cast structure
- nucleation and grain growth
- dendritic growth
- coring & homogenization
- solidification in pure metals and alloys
Cast metal structures
- significance and practical control of cast structure
- grain shape, grain size and orientation
- refinement and modification of cast structure
4.1 INTRODUCTION
The pouring of molten metal into a relatively cool mould initiates the process of solidification
wherein the phase transformation from liquid to solid occurs. The mode of freezing has a twofold
influence upon the final properties of the casting; the metallographic structure acquired by the
casting is determined during solidification, besides structure the soundness of the casting also
depends upon the solidification mechanism.
4.2 MECHANISM OF SOLIDIFICATION
4.2.1 Crystallization of the melt
Crystal lattice represents a more closely packed state of matter than the liquid, thus freezing is
associated with volume contraction. Moreover, freezing results in reduced molecular motion
leading to liberation of energy in the form of latent heat of crystallization which affects the rate
and mode of crystal growth.
Crystallization from the melt involves successive stages of nucleation and grain growth. The
location and relative rates of these two phenomena determines the final structure.
MODULE TWO
SOLIDIFICATION, PHASE DIAGRAM
& STEELS
Module2 4. Solidification
Pruthvi Loy, Chiranth B. P. 2 SJEC, Mangaluru
Figure 4.1: Nucleation and grain growth
At the melting point the thermal fluctuations result in the formation of tiny particles of the
product phase which grow further by the transfer of atoms across its interface. The process of
formation of the first stable tiny particle is called nucleation and the process of increase in the
size of these particles is called grain growth.
4.2.2 Nucleation
Nucleation is the beginning of phase transformation. There can be two types of nucleation:
Homogeneous nucleation and
Heterogeneous nucleation
i) Homogeneous nucleation:
This type of nucleation is observed when solids are formed within its own melt without the aid of
any foreign particles. It occurs in perfectly homogenoeus materials such as pure metals.
Homogeneous nucleation requires some amount of under cooling and the nucleation of super
cooled grains depends on two factores, viz, volume free energy and surface free energy.
a) Volume free energy (fV): the free energy available from the solidifction process and it
depends upon the volume of particle formed.
Where, FV – free energy change per unit volume
(the –ve sign indicates that the free energy decreases)
fV = – 4
3𝜋𝑟3 FV
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Pruthvi Loy, Chiranth B. P. 3 SJEC, Mangaluru
b) Surface free energy (fS): the energy required to form a solid-liquid interface.
Where, – interfacial free energy per unit area
Figure 4.2: Free energy changes in Homogeneous nucleation
Therefore, the total energy change is,
ii) Heterogeneous Nucleation:
It takes place due to the influence of foreign particles (container or insoulble impurities); the
presence of impurites lower the liquid-solid interface energy and help in nucleation there by
reducing the amount of super cooling needed to actuate nucleation.
The basic requirements for hetergeneous nucleations are
a) Nucleating agents: the presence of foreign particles so that nucleation takes place easily
b) Low contact angle: a good wetting between the liquid metal and the foreign particles.
fS = 4𝜋𝑟2
f = – 4
3𝜋𝑟3 FV + 4𝜋𝑟2
Module2 4. Solidification
Pruthvi Loy, Chiranth B. P. 4 SJEC, Mangaluru
Figure 4.3: Contact angle for Heterogeneous nucleation
(with small contact angles the interface has low surface energy and nucleation can occur
at small amount of under cooling)
Once the intial nuclei are established two possiblities exist for further crystallization. i.e., more
solid may be deposited upon the first nuclei or fresh nucleation may occur, but growth might be
expected to predominat over further nucleation. However there could be barriers to growth
resulting from the evolution of latent heat of crystallization and in case of alloys from the change
in compostion of the adjacent liquid through differential freezing.
Table 4.1: Comparison of Homogeneous and Heterogeneous nucleation
Homogeneous nucleation Heterogeneous nucleation
Occurs in pure metals Occurs in alloys
Nucleation is by deposition of atoms from
its own melt
Nucleation is by the influence of foreign
particles
Requires some amount of undercooling
for nucleation.
Undercooling is less as compared to
homogeneous nucleation.
Starts below the equilibrium freezing
temperature
Starts comparatively at much higher
temperature
4.2.3 Grain growth
Growth follows nucleation and it determines the final crystallographic structure of the solid. The
mode of growth depends upon the thermal conditions in the solidification zone and the
constitution of the alloy.
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Pruthvi Loy, Chiranth B. P. 5 SJEC, Mangaluru
Duing growth, the material is transferred by diffusion and the rate of transfer by diffusion can be
analyzed by an equation of Arrhenius type, as shown below;
Where,
Q – Activation energy
R – Gas constant
T – absolute temperature
The growth is controlled by the rate of heat transfer from the casting. Since there is a temperature
gradient towards the casting surface, the growth occurs in a direction opposite to heat flow. For
growth to occur more atoms must join the solid than leave it and for this to happen the
temperature of the interface must be slightely below the equillibrium freezing temperature. This
means some amount of undercooling must exist if the interface is to advance.
Figure 4.4: Material transfer during growth
In case of pure metals, the undercooling can be produced only by thermal means (thermal
undercooling) but in alloys the undercooling may be produced by the changes in temperature as
well as composition which is termed as constitutional undercooling.
Pure metals – thermal undercooling
Alloys – constitutional undercooling
The presence of undercooling alters the growth morphology (i.e., the advance of solidification
front); a faster advancing interface experiences a transition of the solidification front from planar
to cellular and to dendritic from at even higher rates.
Rate of transfer = constant x e - Q/RT
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Pruthvi Loy, Chiranth B. P. 6 SJEC, Mangaluru
Figure 4.5: Transition of growth morphologies due to undercooling
i) Growth in pure metals:
The heat transfer from the casting to the cooler mould produces a positive temperature gradient,
as a result the solid-liquid interface advances progressively as a flat plane under conditions of
slow cooling and steep temperature gradients. Here the latent heat of crystallization is insuficient
to reverse the direction of heat flow.
If the evolution of latent heat is sufficient enough to reverse the temperature gradient at the
interface i.e., a negetive temperature gradient, then minimum temperature in the liquid will no
longer be adjacent to the interface; hence growth by general advance of a smooth solidification
front gives way to other modes of growth.
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Pruthvi Loy, Chiranth B. P. 7 SJEC, Mangaluru
Figure 4.6: Temperature gradient
ii) Growth in alloys:
The growth structure in alloys depends on constitutional undercooling.
Let k0 – be the distribution coefficient.
For k0 < 1, i.e., when the concentration of the solute in the solid is less than that of the liquid,
there must be a rejection of the solute into the liquid at solid-liquid interface and hence the liquid
is enriched with the solute concentration and if sufficient time is not allowed for the solute to
distribute itself throughout the liquid, a concentration gradient is developed which promotes
constitutional undercooling.
Also for k0 > 1, there is a depletion of solute concentration in the liquid adjacent to the interface
which inturn gives rise to constitutional undercooling if sufficient time is not allowed for solute
redistribution.
The variation in concentration of solute in the liquid and solid gives rise to concentration
gradient which in turn gives rise to differential freezing. Differential freezing promotes growth in
a manner other than by the advance of a smooth interface.
𝑘0 = 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝑠𝑜𝑙𝑢𝑡𝑒 𝑖𝑛 𝑡ℎ𝑒 𝑠𝑜𝑙𝑖𝑑
𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝑠𝑜𝑙𝑢𝑡𝑒 𝑖𝑛 𝑡ℎ𝑒 𝑙𝑖𝑞𝑢𝑖𝑑 =
𝐶𝑆
𝐶𝐿
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Pruthvi Loy, Chiranth B. P. 8 SJEC, Mangaluru
Figure 4.7a: Variation of solute concentration (when k0 < 1)
Figure 4.7b: Variation of solute concentration (when k0 > 1)
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Pruthvi Loy, Chiranth B. P. 9 SJEC, Mangaluru
4.2.4 Dendritic growth
Pure metals solidifying with a negative temperature gradient may result in uneven projection of
interface due to the thermal undercooling. The tip of the projection is at a region of greater
undercooling than the remainder of the interface and will have a tendency to grow further into
the liquid. The solids grow in a stem perpendicular to the surface. The latent heat evolved tends
to lower the amount of undercooling at the main interface. The protrusion grows into a spike
while the growth of main interface is somewhat retarded. The spike grows and branches develop
on it, this branched structure is known as a dendrite. The rate of dendritic growth depends upon
the amount of undercooling in the liquid ahead of the advancing dendrite.
Figure 4.8: Dendritic growth structure
In case of alloys as it solidifies, it tends to reject solute at the solid-liquid interface. The rejected
solute elements lower the melting point of the liquid adjacent to the freezing front and tend to
inhibit further solidification. Freezing then continues by dendrites reaching out into the residual
liquid, the dendrite is the result of the preferred growth at an edge or corner of an existing
crystallite. Dendritic growth is most common in alloys (commercial casting alloys forming solid
solution).
Dendritic growth in a pure metal can only be detected by interrupted freezing and decantation,
but is evident in alloys through the persistence of compositional differences, which can be
revealed on etching.
Module2 4. Solidification
Pruthvi Loy, Chiranth B. P. 10 SJEC, Mangaluru
4.2.5 Solidification curves
A solidification curve (or cooling curve) is a record of material temperature vs time, as it cools
down from its molten state to room temperature. It can be used to determine the phase transition
temperatures of materials (pure metals or alloys).
i) Solidification in pure metals
Pure metals have a clearly defined melting or freezing point. i.e., it solidifies at a constant
temperature. The evolution of latent heat is associated with solidification and as a result
temperature remains constant. An equilibrium cooling of pure metal maybe assumed as shown in
figure 4.9. It is also seen that if a pure metal is cooled rapidly or otherwise when it is very pure it
may cool with some amount of undercooling.
Figure 4.9: Cooling curve for pure metals
Skin formation in pure metals:
When pure metals are allowed to solidify in a mould, the portion of the molten metal next to the
mould wall begins to solidify, this metal solidifies in the form of solid skin and then liquid metal
tends to freeze on it. The solid skin progresses towards the center of the mould from mould walls
and due to this progressive solidification, successive layers of molten metal buildup in the form
of solid skin. As the solid metal wall thickness increases the liquid level in the mould falls
because of solidification shrinkage. This causes pipe defects and hence use of risers becomes
necessary.
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Pruthvi Loy, Chiranth B. P. 11 SJEC, Mangaluru
Figure 4.10: Skin formation in pure metals
Also pure metals have high melting temperature as compared to alloys due to which they exhibit
difficulties in casting especially while pouring due to severe metal-mould reactions. Moreover it
is prone to cracking and other defects due to their mode of solidification.
ii) Solidification in alloys
Solidification in solid solution alloys occurs over a range of temperature as shown in figure 4.11
below. In alloys the amount of undercooling required is less as compared to pure metals and
hence the nucleation occurs with ease. A rapidly solidifying alloy may show considerable
amount of undercooling owing to differential freezing.
Figure 4.11: Cooling curve for alloys
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Pruthvi Loy, Chiranth B. P. 12 SJEC, Mangaluru
Alloy solidifying with a mushy zone:
Figure 4.12: Mushy type solidification in alloys
When an alloy solidifies by rejecting solute at the solid liquid interface, the rejected solute
elements lower the melting point of the liquid adjacent to the interface. A mushy zone is thus
formed due to several dendrites reaching out into the residual liquid. Pure metals will have a
narrow mushy zone (steep temperature gradient). Chilling effect brings about steep temperature
gradients thus a narrow mushy zone can be achieved.
4.3 STRUCTURE OF CASTINGS
4.3.1 Significance and control of cast structure
The metallographic structure of a casting consists of
Grain size, shape and orientation
Distribution of alloying elements
Underlying crystal structure and its imperfections
The above stated attributes of a metallographic structure are acquired during solidification and
they define the properties of a casting. These attributes are influenced by various factors such as
the thermal conditions, alloy constitution, conditions for nucleation and growth, etc.
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Pruthvi Loy, Chiranth B. P. 13 SJEC, Mangaluru
The principal factors governing the final metallographic structure are:
1. Casting alloy properties (alloy constitution and thermal properties)
2. Mould properties (design and thermal properties)
3. Solidification process parameters (pouring and casting temperature, conditions for
heterogeneous nucleation, mode of solidification, etc.)
4. Subsequent heat treatments such as annealing, normalizing, etc.
Table 4.2: Influences of casting variables upon structure
Variable Effect Structural tendency
Increasing pouring
temperature
Decreases freezing rate and inhibits nucleation Coarse columnar
Increases temperature gradient Columnar
Decreasing mould
Temperature
Increases freezing rate Fine equiaxed
Increases temperature gradient Columnar
Decreasing pouring
rate
Increases temperature gradient Columnar
Increases mechanical disturbance Fine equiaxed
4.3.2 Grain structure
The grain structure is defined by the grain size, grain shape and its orientation. Grain size is a
very important factor in relation to strength, usefulness and other physical properties. A fine
grained structure offers better strength over a coarse grained structure but suffers low ductility.
The other advantages of a finer grained structure includes
increased impact toughness
improved machining finishes
mitigate cracks and quenching distortions
The grain size in the final product phase depends on the relative rates of nucleation and growth.
As each nucleating particle becomes a grain in the final product; a high nucleation rate means a
large number of grains, moreover with low growth rate more time is available for further
nucleation to take place in the parent phase. Thus with a high nucleation rate and low growth rate
a fine grained structure can be obtained.
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Pruthvi Loy, Chiranth B. P. 14 SJEC, Mangaluru
Typical grain structure
A solidifying melt can assume any of the grain structures as shown in figure 4.13 below.
In case of pure metals a columnar grain structure with the grains growing from the mould wall
towards the casting is observed with a thin layer of equiaxed grains near the mould due to
chilling effect. On the other hand alloys experience better undercooling as compared pure metals
and hence they usually solidify with a completely equiaxed structure.
The metals with minor alloying constituents may solidify with a grain structure intermediate to
pure metals and alloys. The solidification begins similar to pure metals with an equiaxed chill
zone near mould walls and then followed by a columnar zone; as the concentration of the solute
in the remaining melt becomes richer the mode of solidification switches to that of an alloy type
due to increased undercooling and the remaining melt solidifies as equiaxed crystals.
Figure 4.13: Typical grain structures in (a) pure metals (b) metals with
minor alloying constituents, and (c) alloys
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Pruthvi Loy, Chiranth B. P. 15 SJEC, Mangaluru
4.3.3 Grain refinement and modification
Grain refinement is a process of refining the grain structure to impart better properties. It is
simply a technique to control the grain size of the cast structure. Grain refinement aims at
reduction or elimination of columnar zone and increase in number of equiaxed grains
Mechanism of grain refinement and modification
Various practical measures employed for grain refinement and modification are
i) variation of cooling rate
ii) chemical treatment of melt
iii) agitation during freezing
iv) subsequent heat treatments
i) Variation of cooling rate
The association of rapid cooling with fine grain size arises from the influence of undercooling on
the comparative rates of nucleation and growth. Higher nucleation rate is observed due to better
undercooling and less time is available for the grain growth.
ii) Chemical treatment of the melt
Effective grain refinement can be achieved by inoculation which involves the addition to the
melt of small amount of substances designed to promote nucleation. The effectiveness of
inoculants or refiners are determined by their structural affinity (i.e., similarity of symmetry and
lattice parameter)
Example: Silica, Aluminium, titanium are used as refiners during casting of steel.
iii) Agitation during freezing
Nucleation can be brought about by physical disturbance of undercooled liquid, for example by
stirring or gas evolution.
This may be attributed to
a) the widespread distribution of nuclei originally produced at the surface
b) the fragmentation of already growing crystals.
Vibration too is a well-known method of structural refinement. Other practical possibilities
include electromagnetic stirring with the aid of induction coils.
iv) Subsequent heat treatment
With the above techniques the grain size control can be achieved during the casting stage but if
they are found ineffective then the cast specimens may be subjected to subsequent heat treatment
operations such as annealing, normalizing, etc.
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Pruthvi Loy, Chiranth B. P. 16 SJEC, Mangaluru
References:
1. A text book of Foundry Technology – O.P. Khanna
2. Castings - ASM Handbook, Volume 15
3. Foundry Technology – Peter Beeley
4. Castings – John Campbell
5. Material Science & Metallurgy – K.R.Phaneesh
6. Materials Science for Engineers – James F. Shackleford